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Corn flea beetles (Chaetocnema pulicaria Melsheimer) are very small black beetles (Figure 1, top) that can scarify seedling corn leaves very quickly when their populations are high and growing conditions are poor (wet and cold). The scarifying, which appears as white or grayish streaks, removes the photosynthetic producing green layer.
Direct leaf feeding is usually not a major concern unless the plants are under stress and unable to outgrow the feeding damage. Good growing conditions can allow the plants to establish new leaves and continue development. A greater economic concern is the potential transmission of Stewart’s wilt bacterium as they feed (Figure 1, bottom). The transmission is a result of feeding on infected plants before overwintering in nearby grassy areas. Seed products without resistance or very good tolerance to Stewart’s bacterial wilt can wilt and die. Seed treatments with insecticidal properties and soil applied insecticides at planting can help manage flea beetles.
Figure 1. (Top) Flea beetle. Photo courtesy of Frank Peairs, Colorado State University, Bugwood.org. (Bottom) Stewart’s wilt as a result of overwintering adult corn flea beetle feeding. Photo courtesy of Jerald Pataky, University of Illinois, Department of Crop Science.
If flea beetles or their feeding scars are found during routine seedling scouting, a more in depth evaluation should be completed. The beetles jump quickly to the soil or other plants when disturbed; therefore, move slowly around the plants or their presence may be missed. To determine populations, randomly select 20 consecutive plants in 5 field areas. Cautiously observe the top and undersides of leaves for live beetles. Record the number of damaged plants, the severity of damage, and the number of beetles.
Controls may be necessary if seedlings have less than 4 collared leaves showing (V4 growth stage), 50% of the seedlings show severe feeding, presence of 5 or more beetles/plant, and growing conditions are poor. Mild winters favor adult survival. If the sum of the mean daily temperatures for December, January, and February total above 90, survival can be moderate. Above 100 survival can be high.